Lighttpd - Error while trying to run C application through CGI

Greetings fellow programmers,

I'm a bit new here, so please be gentle.

Now the problem, I'm currently working on a dynamic web application running on WAGO PLC (750-8202) with embedded linux. Lighttpd is installed on the PLC itself. The goal is to monitor/edit I/O of the PLC from the web app.

After a lot of reasearch I found out I need to use CGI to communicate with my C applications that can return the information from the PLC that I need. I tried some simple hello world codes, but always with an error.

Here's my lighttpd.conf

# Common configuration values.
server.document-root    = "/var/www"
server.username         = "www"
server.groupname        = "www"
server.tag              = "lighttpd"
server.errorlog         = "/var/log/lighttpd/error.log"
accesslog.filename      = "/var/log/lighttpd/access.log"

index-file.names        = ( "index.html", "index.php" )
server.modules          = (
    "mod_access",
    "mod_accesslog",
    "mod_cgi",
    "mod_fastcgi",
    "mod_rewrite",
    "mod_redirect",
    "mod_auth",
    "mod_proxy"
)

include "mode.conf"
include "mime_types.conf"
include "mod_fastcgi.conf"
include "auth.conf"
include "redirect_test.conf"

$HTTP["url"] =~ "/cgi-bin/" {
        cgi.assign = ( "" => "" )
}

cgi.assign      = (
        ".cgi"  => ""
)
url.rewrite-once = (
  # Codesys3 webvisu forces the browser to come out with POST requests to the root context.
  # Move that to the /webvisu/ context so it goes through the proxy 8000.
  "^/WebVisuV3.bin" => "/webvisu/WebVisuV3.bin",
  # Redirect all http[s]://<ip>/rest/ URL's to the RESTful API Example "/rest/index.php'  
  "^(?:.*)/rest/(.*)" => "/rest/index.php/$1"
)

# Transfer all http[s]://<ip>/webvisu/ URL's to the proxy server on port 8000.
# That is, all codesys webvisu traffic goes through the proxy.
#$HTTP["url"] =~ "^/webvisu/.*" {
#    proxy.server = ("" => (( "host" => "127.0.0.1", "port" => proxy_port )) )
#}

# Activate proxy server on port 8000. Sends all requests from the browser to
# the codesys webserver (localhost:8080).
$SERVER["socket"] == "127.0.0.1:" + proxy_port {
    url.rewrite-once = (
        "^/webvisu/$" => "/webvisu.htm",
        "^/webvisu/(.*)" => "/$1"
    ),
    proxy.server = ( "" => (( "host" => "127.0.0.1", "port" => 8080 )) )
}

And now a simple application I'm trying out written in C:

#include <stdio.h> 
int main(void) 
{ 
printf("Content-Type: text/plain;charset=us-ascii\n\n");
printf("Hello world\n\n");  
return 0; 
}

I call my application like this:

http://192.168.1.2/cgi-bin/foo.cgi

And only get a blank page. If I check error log, it gives me this error:

(mod_cgi.c.1341) cleaning up CGI: process died with signal 6 

Can anyone point out where I'm making some kind of mistake? Or am I doing it all wrong? Is there some other, better way to communicate with C apps? I lost several days trying to figure it out and now I'm really let down. Any kind of help will be greatly appreciated.

Thanks a lot in advance and have a nice day!